1. Field of the Invention
This invention relates to a liquid crystal alignment agent, more particularly to a liquid crystal alignment agent including a hydroxyl compound with a specific weight average molecular weight range. The invention also relates to a liquid crystal alignment film formed from the liquid crystal alignment agent, and a liquid crystal display element including the liquid crystal alignment film.
2. Description of the Related Art
Due to an increasing demand for liquid crystal display devices with a wide viewing angle characteristic, liquid crystal display devices are being developed toward high performance, and thus the requirement for good electrical and/or displaying qualities in terms of liquid crystal alignment property, voltage holding ratio, ion density, image sticking, or the like has become stricter. It is also desirable to maintain such good qualities during long-term use. Among the aforesaid qualities, voltage holding ratio and ion density are of primary concern. When the voltage holding ratio of a liquid crystal display device is undesirably reduced, the displaying quality cannot satisfy the requirement for high contrast and high light transmission. On the other hand, when the ion density is undesirably increased, the image sticking problem may occur.
WO 2008/078796 discloses a liquid crystal alignment film having a high voltage holding ratio and a low ion density, and an N-substituted diamine compound used for preparing the liquid crystal alignment film. The N-substituted diamine compound is represented as follow:

where Ra1 represents a C1-C5 alkylene group, Ra2 and Ra1 independently represent hydrogen or a C1-C4 alkyl group, and at least one of Ra2 and Ra3 is a C1-C4 alkyl group. The problems of inferior voltage holding ratio and undesirably high ion density encountered in conventional liquid crystal display devices can be improved by using the N-substituted diamine compound for preparing the liquid crystal alignment film. However, the liquid crystal alignment film made by using the N-substituted diamine compound still has the problems of reduced voltage holding ratio and increased ion density after a period of exposure to ultraviolet (UV) irradiation. Problems such as reduced contrast and image sticking may also arise when the liquid crystal alignment film disclosed in WO 2008/078796 is used in a liquid crystal display device.
There is an increasing demand in the art for a liquid crystal alignment agent having superior UV stability so that a high voltage holding ratio and a low ion density of a liquid crystal display device employing a liquid crystal alignment film formed from the liquid crystal alignment agent can be maintained even after long-term UV irradiation.